Study helps pinpoint cirrus cloud formation

Cirrus cloud study helps inform climate predictions.

Composition of seed material suggests human activity could be a significant factor

By Summit Voice

SUMMIT COUNTY — Mineral dust and metallic aerosols are the key seeding agents for the formation of high-altitude cirrus clouds, which cover nearly a third of the globe at any given time. Often forming more than 10 miles up, cirrus clouds can cool the planet by reflecting solar radiation, and warm it, by trapping heat like a blanket.

A nine-year study of cirrus clouds using using instruments aboard high-altitude research aircraft is helping scientists get a better handle on the mechanisms driving cirrus cloud formation, and that, in turn, could help scientists predict future climate patterns.

“We think we’re really looking at the seed, the nucleus of these ice crystals,” said Dan Cziczo, an associate professor of atmospheric chemistry at MIT. “These results are going to allow us to better understand the climatic implications of these clouds in the future.”

Cziczo and his colleagues published their results this week in Science.

The research team conducted four flight missions between 2002 to 2011, in regions of North America and Central America where cirrus clouds often form. Before takeoff, the team received weather forecasts, including information on where and when clouds might be found.

“More often than not, the forecast is solid, and it’s up to the pilot to hit a cloud,” Cziczo says. “If they find a good spot, they can call back on a satellite phone and tell us if they’re inside a cloud, and how thick it is.”

For each mission, Cziczo and Karl Froyd, of NOAA‘s Earth System Research Laboratory, mounted one or two instruments to the nose of each plane: a single particle mass spectrometer and a particle collector. As the research planes sped through the clouds gathering particles, the on-board instruments measured the kernels and assessed them for composition.

The human factor

After additional analysis in the lab, the scientists concluded that more than 60 percent of cloud particles consisted of mineral dust blown into the atmosphere, as well as metallic aerosols, suggesting there’s a human factor, with sources originating from agriculture, transportation and industrial processes also release dust into the atmosphere.

“Mineral dust is changing because of human activities,” Cziczo said. “You may think of dust as a natural particle, but some percentage of it is manmade, and it really points to a human ability to change these clouds.”Some global-modeling studies predict higher dust concentrations in the future due to desertification, land-use change and changing rainfall patterns due to human-induced climate effects,” he added.

The researchers also found metallic compounds, including lead, zinc and copper, that may point to a further human effect on cloud formation.

“These things are very strange metal particles that are almost certainly from industrial activities, such as smelting and open-pit burning of electronics,” Cziczo said. Lead is also emitted in the exhaust of small planes.

Contrary to what many lab experiments have found, the team observed very little evidence of biological particles, such as bacteria or fungi, or black carbon emitted from automobiles and smokestacks. Froyd says knowing what particles are absent in clouds is just as important as knowing what’s present: Such information, he says, can be crucial in developing accurate models for climate change.

“There’s been a lot of research efforts spent on looking at how these particle types freeze under various conditions,” Froyd says. “Our message is that you can ignore those, and can instead look at mineral dust as the dominant driving force for the formation of this type of cloud.”

This story was adapted from an article written by Jennifer Chu, with the MIT news office.